This invention relates to a method of drilling a bore hole and more particularly to a method of steering the drill bit while drilling a curved bore hole to vary the azimuth.
A method is disclosed in U.S. Pat. No. 5,215,151 (Smith et al) in which the drilling of a bore hole is effected using continuous coiled tubing which extends from a trailing end on a supply reel at the earth's surface to a leading end within the well bore.
The drilling of well bores using continuous coiled tubing is known conventionally and includes the supply of a drilling fluid which is pumped into the trailing end of the coiled tubing for transmitting the drilling fluid to the leading end of the tubing at the base of the well bore. At the base is provided a drilling tool which includes a drill bit rotatable relative to the drilling tool, the drill bit being driven by a motor powered by the flow of the drilling fluid through the drilling tool.
It is further previously known that a substantially vertical well bore can be turned with a curved bore section into an inclined or horizontal well bore by providing a drilling tool which includes a bend section. The bend section of the drilling tool tends to steer the well bore so that it turns to a direction at right angles to a plane containing the bend axis and longitudinal of the drilling tool.
The above U.S. patent discloses a technique of steering the drilling tool to vary the azimuth of the curved bore section by providing an orientation device as a part of the drilling tool. The drilling tool thus comprises an upper part fixed relative to the drill tubing and a lower part including the drill bit and the bend section. A control system is provided by which the lower section can be rotated relative to the upper section by controlled predetermined amounts in response to signals provided from the surface.
This arrangement is generally satisfactory and has achieved some success but is relatively complex involving signaling from the surface and relatively complex mechanical structures in the drilling tool.
It has also been proposed to steer the drilling tool by rotating the injector about the axis of the drill string. This acts to rotate the tubing which in turn rotates the drilling tool to the required angle. However this has only been considered in an arrangement in which the reel and reel support is also rotated about the same axis. As the reel is heavy and offset to one side of the drill string, this rotation has been considered to be impractical and the technique basically abandoned.